"Koga's Zero" is the Mitsubishi A6M2 fighter recovered almost intact in the Aleutians in 1942. The plane was sent to San Diego for analysis and testing. It was thoroughly restored, painted and marked as a U.S. aircraft, then extensively flight tested, including one-on-one sorties against various USN and USAAF fighters.
This page includes links to several photos of the plane, plus two original flight test reports.
(Thanks to Rich Leonard for these photos and documents.)